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Geochemistry of Carboniferous peralkaline felsic volcanic rocks, central New
Brunswick: examination of uranium potential
Taryn R. Gray1*, Jaroslav Dostal1, Malcolm McLeod2, Duncan Keppie3, Yuanyuan
Zhang4
1
Department of Geology, Saint Mary’s University, Halifax, Nova Scotia, B3H 3C3,
Canada <[email protected]>
2
New Brunswick Department of Natural Resources and Energy, Geological Surveys
Branch, P.O. Box 5040, Sussex New Brunswick E4E 5L2, Canada
3
Instituto de Geología, Universidad Nacional Autónoma de México, 04510, México
4
Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School
of Earth and Space Sciences, Peking University, Beijing, China 100871
*corresponding author
ABSTRACT
Felsic volcanic rocks from the ca. 335 Ma Cumberland Hill Formation of southern
New Brunswick, Canada, consist of peralkaline rhyolite and trachyte. These rocks are
interpreted to have undergone extensive fractional crystallization from an alkali basaltic
magma, although their different geochemical signatures indicate that the rhyolite was not
derived from the trachyte via continuous fractional crystallization. The rhyolite is highly
enriched in incompatible trace elements including uranium (up to ca. 20 ppm).
Cumberland Hill peralkaline rhyolite suggests that a potentially economic source of
uranium exists in the basin, based on the elevated uranium concentrations and high
potential for remobilization into surrounding sedimentary units.
INTRODUCTION
Igneous rocks are sparsely distributed throughout the Middle Devonian-Permian
Maritimes Basin (Fyffe and Barr 1986; Dunning et al. 2002). They appear to decrease in
volume through time from: (i) Middle to Late Devonian tholeiite, minor alkali basalt,
bimodal gabbro and A-type to evolved I-type granite; (ii) Lower Carboniferous bimodal
igneous rocks, and most sparsely, (iii) Upper Carboniferous bimodal igneous rocks (New
Brunswick Department of Natural Resources 2010). The felsic igneous rocks, as well as
the interbedded lithic sandstone, conglomerate, and feldspathic sandstone of the
Cumberland Hill Formation, the topic of this paper, were considered part of the Late
Wesphalian Pictou Group, and consequently associated with the latter episode of
magmatism based on stratigraphic relationships (Fyffe and Barr 1986). However, recent
mapping indicated these volcanic rocks lie within the Viséan to early Namurian Mabou
Group, and therefore are associated with the middle episode of volcanic activity (St. Peter
1997; New Brunswick Department of Natural Resources 2010). A mid-Viséan U-Pb
zircon date of 335 ± 2 Ma age substantiates this interpretation (St. Peter 2002). Hence the
felsic volcanic rocks in the Cumberland Hill Formation appear to have approximately the
same age as several alkali mafic units in New Brunswick (e.g., the Hardwood Ridge,
Royal Road, and Queenstown basalts), and the tholeiitic-alkali mafic rocks of the Cap
aux Diables Formation in the Magdalen Islands (Barr et al. 1985; Fyffe and Barr 1986;
LaFleche et al. 1998; New Brunswick Department of Natural Resources 2010).
The mafic rocks have continental within-plate characteristics (Barr et al. 1985), and
HIMU-OIB Pb-isotopic signatures that have been related either to a mantle plume (PePiper and Piper 1998) or to decompression melting below a pull-apart rift (LaFleche et al.
1998). However, the plume model (Murphy et al. 1999 and Keppie and Krogh 1999) is
inconsistent with U-Pb chronology of Devonian to early Carboniferous volcanics within
the southern Magdalen Basin (Dunning et al. 2002).
In this paper, we selected key samples from the Cumberland Hill Formation in order to
petrologically and geochemically characterize the rocks and compare the rhyolite and
trachyte to similar rocks within the Maritimes Basin. This is critical when accessing the
uranium potential within Cumberland Hill, as similar rocks within the Maritimes Basin
have been identified as hosts of economic uranium mineralization (e.g., Dahlkamp 1993;
Plant et al. 1999; Cuney 2009; Nash 2010).
GEOLOGICAL SETTING
Late Devonian to Early Permian rocks of New Brunswick comprise the western part
of the Maritimes Basin of Atlantic Canada. The Maritimes Basin is a successor basin
within which there are a number of sub-basins separated by fault-bounded ridges of
basement (Fig. 1a). In New Brunswick, these rocks occur within deep depositional
centres or subbasins, or on shallowly buried or partially exposed basement uplifts and
platforms (e.g., St. Peter and Johnson 2009).
The volcanic rocks of the Cumberland Hill Formation crop out as several inliers and
are unconformably overlain by the Upper Carboniferous Pictou Group on the New
Brunswick Platform (Fig. 1b). The outcrops are likely related to a single volcanic centre
partially hidden beneath Pictou Group rocks, because the area incorporating the inliers
clearly defines a circular area of high magnetic response on aeromagnetic maps (Thomas
and Kiss 2005).
Fyffe and Barr (1986) examined some of the felsic volcanic rocks in the vicinity of
Cumberland Hill, including lava flows and tuffs in a rhyolitic unit and underlying
trachytic unit. Subsequently, St. Peter (1997) delineated the distribution of these units in
more detail, depicting the trachyte as mostly underlying, but in part interdigitating with,
the rhyolite. There is also one massive intrusive porphyry plug with several smaller
dykes/pipes, interpreted by Thomas and Kiss (2005) as volcanic feeder dykes, based on
their bulls-eye positive magnetic anomalies (50-375 nT) within the magnetically
anomalous area. The exact thickness of these units is unknown as they are poorly
exposed, however, both trachytic and rhyolitic units are believed to be tens of metres
thick (St. Peter 1997).
PETROGRAPHY
The rhyolite occurs as aphanitic to porphyritic flows and ignimbritic tuffs with flow
banding preserved and microporphyritic to porphyritic textures (Fig. 3a). Flow alignedphenocrysts of feldspar (mainly anorthoclase; Fyffe and Barr 1986; 0.1-0.5 mm in length)
and quartz (0.3-1 mm) are enclosed in a quartzfeldspathic groundmass with rare green
clinopyroxene (hedenbergite; Fyffe and Barr 1986). The groundmass is composed of
quartz, feldspar, devitrified glass, minor magnetite, ilmenite, pyrite, rare galena, fluorite
and accessory minerals similar to those in trachyte samples (i.e. microlites of zircon,
apatite, monazite and allanite). The rhyolite is also extensively altered, although to a
lesser degree than the trachyte, with most primary magmatic phases replaced by a
secondary assemblage (i.e., carbonate, sericite, albite, and chlorite).
The trachyte occurs in two varieties, porphyritic and aphanitic, the latter of which is
dominant. The porphyritic trachyte contains phenocrysts of feldspar (sanidine or
anorthoclase) up to 2 mm in size, rare clinopyroxene, and plagioclase (Fig. 3b). The
groundmass is largely composed of devitrified glass with phenocrysts of feldspar,
clinopyroxene, and quartz, and rare ilmenite, hematite, sphalerite, and galena. Accessory
minerals (fine-grained microlitic zircon, apatite, monazite and allanite) were identified
and confirmed using a scanning electron microscope including backscattered electron
imaging, energy-dispersive X-ray spectroscopy and X-ray element mapping. The
aphanitic variety displays a typical trachytic texture with flow-aligned feldspars (0.1-0.5
mm in length) enclosed in a fine-grained matrix similar to that of the porphyritic type.
Both varieties are pervasively altered, with only relicts of primary minerals present, and
secondary minerals dominated by calcite, sericite, chlorite, albite and hematite alteration.
GEOCHEMISTRY
Fifteen samples were collected from the Cumberland Hill Formation, crushed and
pulped in a soft iron swing mill, then analyzed for major elements using glass discs by Xray fluorescence spectrometry in the Nova Scotia Regional Geochemical Centre at Saint
Mary’s University, Halifax. Samples were subsequently analyzed for trace element
compositions using a Perkin Elmer Optima 3000 inductively coupled plasma-mass
spectrometer (ICP-MS) in the Activation Laboratories in Ancaster, Ontario. Prior to
analysis, samples were digested in nitric acid after lithium metaborate fusion to ensure
the complete dissolution of refractory accessory minerals. Precision and accuracy of the
X-ray fluorescence data are reported by Dostal et al. (1995). The analytical error of the
trace element determinations is 2–10%.
According to the Na2O+K2O vs SiO2 classification of LeMaitre et al. (1989; Fig. 4a)
and the Zr/TiO2 vs SiO2 diagram (Fig. 4b) of Winchester and Floyd (1977), the rocks can
be subdivided into two groups, trachyte and peralkaline rhyolite, analogous to
comendite/pantellerite fields according to Winchester and Floyd (1977). Most rocks are
peralkaline with a high agpaitic index (AI = mole (Na+K)/Al; Shand 1951) and acmite in
their CIPW normative mineralogy. Considering loss of ignition (LOI)-free analyses
(Table 1), the trachyte on average (n=7, 1) has moderate SiO2 (65  2.48 wt. %)
accompanied by high total alkalis (9.5  1.81 wt. %), low CaO (2.5  1.21 wt. %), high
FeOtot/MgO (ca. 25) and elevated concentrations of high field strength elements (HFSE)
and other incompatible elements (e.g., Zr+Nb+Ce+Y = 1334  409.70; Whalen et al.
1987). The average of the rhyolitic rocks (n=8, 1) has higher SiO2 (75.0  0.66 wt. %),
equivalent total alkali concentrations (9.0  0.29 wt. %), very low CaO (0.23  0.08 wt.
%), high FeOtot/MgO (ca. 140), and high HFSE and other incompatible elements (e.g.,
Zr+Nb+Ce+Y = 3126  161.75; Whalen et al., 1987).
The major and trace element signatures of the volcanic rocks of the Cumberland Hill
Formation are interpreted to vary predominantly due to the degree of fractionation.
Conversely, a heterogeneous magma source could also contribute to the variations
exhibited. In order to determine the fractionation trends, Nb was utilized as a
differentiation index following White et al. (2006; Fig. 5). With increasing concentration
of Nb, the trachyte samples display positive correlation with Al2O3, and negative
correlation with TiO2. The reduction in TiO2 concentration can most likely be attributed
to crystallization of Fe-Ti oxides. Rhyolite samples, in contrast, have relatively constant
Al2O3 (Fig. 5a) and TiO2 (Fig. 5b). Trace elements also display different fractionation
trends in trachyte and rhyolite samples. Trachyte samples show marked decreases in Ba
with increasing Nb (Fig. 5c), reflecting the crystallization of feldspars, as well as an
increase in incompatible trace elements including Zr, La, and Ga (Fig. 5d, e, f). However,
the abundances of these trace elements in the rhyolite remain virtually unchanged with
increasing Nb. The contrast in fractional crystallization between trachyte and rhyolite is
highlighted by the CaO-K2O-Na2O ternary diagram (Fig. 6), which illustrates that
trachyte samples plot in a trend away from the CaO apex, whereas rhyolite sample
variation is parallel to the K2O-Na2O join. This difference is most readily explained by
the crystallization of Ca-bearing plagioclase and clinopyroxene in the trachyte magma,
and alkali feldspar crystallization in the rhyolite magma.
The chondrite-normalized rare earth element (REE) patterns for each suite (Fig. 7a)
are strongly enriched in light REE (LREE). The rhyolite has higher LREE contents with
Lan values of ca. 440-510 compared to the trachyte with Lan ca. 270-430. The trachyte
shows steeper LREE to heavy rare earth element (HREE) profiles, with Lan/Ybn ratios ca.
8-11 compared to ca. 4-6 in rhyolite. A similar relationship is observed for Lan/Smn,
which in the trachyte is ca. 2.6-3.4 and in the rhyolite is ca. 2.2-2.6. The shape of the
REE patterns in the trachyte does not change significantly, although the absolute
abundances are escalating with the increasing degree of differentiation. The REE patterns
of the rhyolite are characterized by a pronounced negative Eu anomaly (average Eu/Eu*
=0.08), consistent with low-pressure fractional crystallization of alkali and/or plagioclase
feldspar. Primitive mantle-normalized trace element patterns in both rock types (Fig. 7b)
are fairly similar. Both are highly fractionated and peak at Th-Nb; however, the rhyolite
displays strong, negative Ba, Sr, and moderately negative Eu anomalies, whereas the
trachyte contains only slightly negative anomalies in these elements. The Nb/Ta ratios of
the trachyte (14.27  0.47) and rhyolite (13.91  2.48) are the same, but slightly lower
than that of a mantle (typically ~ 17.5; Sun and McDonough, 1989).
The abundances of U and Th in the felsic rocks of the Cumberland Hill Formation are
high and variable. Rhyolite display significantly higher contents of U (14.0  4.0 ppm,
n=8), and Th (33.0  3.1 ppm, n=8) compared to trachyte (U= 2.4  2.4 ppm, n=7; Th=
12  1.4 ppm, n=7). The large range in U may be related to devitrification and alteration
of the rhyolite groundmass. The rhyolite samples also have a higher U/Th ratio (0.44 
0.13, n=8) than trachyte (0.32  0.09, n=7), which is comparable to the whole rock or
melt inclusion data from non-mineralized peralkaline rhyolite from various U districts in
the world (Cuney and Kyser 2009). The increase of U/Th ratio in the rhyolite compared
to trachyte may reflect monazite fractionation. Based on the variability of the U and Th
concentrations in the rhyolite and trachyte samples, we interpret that remobilization of
these elements has occurred. This remobilization was likely caused by devitrification of
the groundmass because zircon is modally insignificant, and not the principal host of
these elements based on the lack of correlation of radioactive trace elements with Zr (see
Dupuy and Dostal 1983).
PETROGENESIS
Overall the geochemical and mineralogical similarities of the spatially and temporally
associated trachyte and rhyolite imply they are genetically related. However,
discontinuous and/or contrasting variation trends and significant differences in
incompatible trace element ratios indicate that the rhyolite was not derived from the
trachyte via continuous fractional crystallization, although fractional crystallization may
be responsible for the variations within the individual suites. The evolution of the
trachyte in the melt phase involved crystallization of feldspars, clinopyroxene, and Fe-Ti
oxides, whereas the evolution of the rhyolite was dominated by crystallization and
fractionation of alkali feldspar.
An alternative model to continuous fractional crystallization for the origin of the
rhyolite is assimilation-fractional crystallization. However, the low and overlapping
Th/Ta ratio of the trachyte and rhyolite (Fig. 8a), a sensitive indicator of crustal
contamination (Gorton and Schandl 2000), demonstrates that assimilation-fractional
crystallization and crustal contamination processes cannot solely account for derivation
of the rhyolite from the trachyte. In addition, Pe-Piper and Piper (1998) reported an εNd
value of +3, with an assumed age of 310 Ma from the Cumberland Hill rhyolite. This
relatively high value implies that significant assimilation of crustal material is not likely
to have taken place.
K/Rb ratios in trachyte samples are much higher than in rhyolite samples (Fig. 9a, b),
which suggest the evolution of the rhyolite may have been affected by fluids (Dostal and
Chatterjee 1995). Again, this mechanism cannot explain the origin of the rhyolite, but
rather indicates that fluids played an important role during the evolution of these rocks
and perhaps even led to the mobilization of U (Cuney and Kyser 2009; Nash 2010) in the
rhyolite (Fig. 9b).
A comparison with similar rocks elsewhere (e.g., Pecerrillo et al. 2003) suggests that
the Cumberland Hill trachyte and rhyolite could represent various stages of fractional
crystallization in an evolving alkali basaltic magma. The absence of a negative Nb
anomaly suggests that the parent magmas were not derived from subcontinental
lithospheric mantle, but more plausibly originated from asthenospheric mantle. This
interpretation is supported by the rhyolite εNd value of +3 (Pe-Piper and Piper 1998),
which imply upward migration of enriched asthenospheric magma into a juvenile
lithosphere.
LaFleche et al. (1998) and Pe-Piper and Piper (1998) described alkali basaltic rocks of
comparable age from other parts of the Maritimes basin. The presence of alkali gabbroic
rocks of a similar age (Johnson 2008) in the area suggests that the felsic rocks of the
Cumberland Hill Formation could be derived from such parent magma by fractional
crystallization. In fact, peralkaline felsic volcanic complexes are typically associated with
a shallow-seated alkali intrusions ranging in composition from gabbro to highly
fractionated granitic rocks. Although geochemical data are insufficient from the mafic
alkali rocks in the vicinity of the Cumberland Hill Formation, chemical analyses of alkali
basaltic rocks reported by LaFleche et al. (1998) from the Magdalen Islands indicate that
similar basalts could have been a parent to trachyte and rhyolite. Trachyte of similar
compositions has been documented to evolve from alkali basalt by extensive fractional
crystallization (Peccerillo et al. 2003) and peralkaline rhyolite could have evolved from
such magma by further fractional crystallization in a subvolcanic magma chamber. A
likely scenario for the Cumberland Hill felsic rocks is that they are the products of
fractional crystallization of alkali basaltic magma, and that the trachyte and rhyolite
represent separate magma pulses with the latter being younger having undergone more
extensive fractionation prior to emplacement.
ECONOMIC POTENTIAL
Silica-oversaturated peralkaline felsic volcanic rocks, particularly rhyolite, are
commonly enriched in U and represent a potential source of uranium for volcanic-related
hydrothermal U deposits, such as the Streltsovka caldera (Transbaikalia, Russia:
Ischukova 1997; Chabiron et al. 2001, 2003; Cuney and Kyser 2009; Nash 2010),
sediment-hosted U deposits in siliciclastic sedimentary basins, such as the U deposits in
the Tim Mersoi basin in Niger (Forbes et al. 1984; Forbes 1989; Plant et al. 1999; Pagel
et al. 2005; IAEA 2009), and the New Horton mineralization (U-Cu-Ag-Au) within the
Carboniferous basin (Boyd 1978; McLeod and Smith 2010; New Brunswick Department
of Natural Resources 2010).
The crystallization of U-bearing accessory minerals in highly depolymerized
peralkaline melts is typically suppressed (Cuney 2009; Cuney and Kyser 2009) leading to
progressive enrichment of U in residual melts during fractional crystallization and
entrapment of U in the glassy groundmass. Subsequently U can be leached during the
alteration or devitrification of the glass (Nash 2010). Cuney (2009) and Cuney and Kyser
(2009) inferred that ignimbritic tuffs, in which U is hosted in the glass, are the most
favourable type of volcanic rock; such rhyolitic pyroclastic rocks are abundant in the
Cumberland Hill Formation.
Rhyolite of the Cumberland Hill Formation has relatively high abundances of U
compared to associated trachyte and could be a source of U mineralization. The U
concentrations in these rhyolite samples are elevated compared to similar felsic and mafic
Late Tournaisian–Early Viséan deposits of the Maritimes Basin (Magdalen Islands, 0.41.9 ppm; Central New Brunswick, 0.8-1.9 ppm and the Cobequid Highland dykes, 0.2
ppm; Pe-Piper and Piper 1998). The abundances of U in the rhyolite are comparable to
the non-mineralized U concentrations of the peralkaline rhyolite of the Streltsovka
caldera (Fig. 7b), the largest, volcanic-related, U ore field in the world (Cuney 2009).
Despite similar U concentration, it is important to consider where U is hosted in rhyolite.
When U is hosted in zircon or other resistant accessory minerals it cannot be readily
remobilized. Based on current geochemical work, it appears U is present within the
glassy groundmass of the rhyolite in the Cumberland Hill Formation and can therefore be
released and re-deposited and thus could be a source of U within U- (Cu-V-Ag) deposits
within local parts of the Carboniferous basin.
CONCLUSIONS
Felsic volcanic rocks from the Cumberland Hill Formation consist of peralkaline
rhyolite and trachyte. Both contain elevated concentrations of incompatible trace
elements, with the rhyolite more enriched and fractionated than the trachyte. Although
both rock types appear to be genetically related, differences in their geochemical
signatures imply the rhyolite was not derived from the trachyte by continuous fractional
crystallization. Examination of the Th/Ta ratio also indicated crustal contamination
cannot account for the observed compositional contrast. A preferred model is that the
trachyte and rhyolite originated from the same or highly similar parent magma, and
represent separate magma pulses with the latter being younger, having undergone more
extensive fractionation prior to emplacement. The absence of a negative Nb anomaly
suggests the parent magma was not derived from subcontinental lithospheric mantle, but
more plausibly originated from an asthenospheric mantle source. Lastly, Cumberland Hill
rhyolite contains elevated U concentrations, with U hosted within the glassy groundmass.
Since U can be leached and re-deposited, the rhyolite could be a source of sediment
hosted U mineralization within the basin.
ACKNOWLEDGEMENTS
A grant from the Geological Survey Branch of the New Brunswick Department of
Natural Resources and Energy (to JD) covered fieldwork expenses. Analytical work was
predominantly funded by a Natural Sciences and Engineering Research Council of
Canada (NSERC) Discovery Grant (to JD), and in part by a Society of Economic
Geologists Student Research Grant (to TRG). The manuscript benefits from the
constructive reviews by David Lentz, an anonymous reviewer, and by journal editor
Sandra Barr
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St. Peter, C. J. 1997. Bedrock geology of Chipman – Canaan River map area (parts of
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St. Peter, C.J. 2000. Carboniferous geology of the southwestern New Brunswick platform
(Maugerville Subbasin). New Brunswick Department of Natural Resources and
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St. Peter, C.J., 2002 (unpublished). Bedrock Nomenclature of New Brunswick:
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FIGURE CAPTIONS
Figure. 1. (a) Map of Atlantic Canada depicting the Maritimes Basin, adapted from Roliff
(1962), Williams (1974), Bradley (1982) and Fyffe and Barr (1986). Major faults are
indicated as dashed lines; study area is indicated with a box, (b) Geological map of the
Cumberland Hill area, southern New Brunswick (adapted from Smith 2007) indicating
the areal extent of the Cumberland Hill trachyte and rhyolite both of which are
interbedded with lithic and feldspathic sandstone and conglomerate, and contain
interdigitating feldspar porphyry, rhyolite ash tuff, breccia and lapilli tuff (St. Peter
2002). The Hurley Creek Formation is composed of mudstone, fine-grained sandstone,
coarse-grained feldspathic and lithic sandstone and polymictic pebble conglomerate at the
top (Muller 1951). The Minto Formation contains feldspathic, pebbly and fine-grained
sandstone, polymictic conglomerate and minor laminated mudstone, shale and thin coal
seams (Dyer 1926; Muller 1951; St. Peter 2000).
Figure 2. Carboniferous stratigraphy in the central New Brunswick Basin (adapted from
St. Peter 1997). The Cumberland Hill and Hardwood Ridge Formations are part of the
Mabou Group, wheresas the Minto Formation, Hurley Creek Formation and Sunbury
Creek Formation belong to the Pictou Group.
Figure 3. Whole rock slab photos indicating a) flow banded rhyolite texture, with a
quartzfeldspathic groundmass, and b) Vuggy trachyte with chlorite infilling.
Figure 4. Plots of (a) Na2O+K2O versus SiO2 (wt. %; Le Maitre et al. 1989), (b) Zr/TiO2
versus SiO2 (wt. %; Winchester and Floyd 1977), and (c) Zr/TiO2 versus Nb/Y
(Winchester and Floyd 1977) for volcanic samples from the Cumberland Hill Formation.
Figure 5. Plots of Nb (ppm) versus (a) Al2O3 (wt. %), (b) TiO2 (wt. %), (c) Ba (ppm), (d)
Zr (ppm), (e) La (ppm), and (f) Ga (ppm) for volcanic samples from the Cumberland
Hill Formation
Figure 6. CaO-K2O-Na2O ternary diagram for volcanic samples from the Cumberland
Hill Formation. Trends indicate that fractionation of Ca-plagioclase and clinopyroxene
could have occurred in trachyte, while alkali feldspar fractionation is more likely in
rhyolite.
Figure 7. (a) Chondrite-normalized REE patterns of the averages of trachyte (n=7) and
rhyolite (n=8) samples from the Cumberland Hill Formation and rhyolite from
Streltsovka (n=1; Chabiron et al. 2003); (b) Primitive mantle-normalized multi-element
plots of the average of trachyte (n=7) and rhyolite (n=8) samples from the Cumberland
Hill Formation and rhyolite from Streltsovka (n=1; Chabiron et al. 2003). Normalizing
values are after Sun and McDonough (1989).
Figure 8. Discrimination diagrams for felsic volcanic rocks from the Cumberland Hill
Formation. (a) Yb (ppm) versus Th/Ta (Gorton and Schandl 2000). Abbreviations:
WPVZ = within plate volcanic zones; MORB = mid-ocean ridge basalt; WPB = withinplate basalt. (b) Rb vs Nb+Y and (c) Ta vs Yb (Pearce et al. 1984). Abbreviations: synCOLG = syn-collisional granite; WPG =within-plate granite; VAG = volcanic-arc
granite; ORG = oceanic-ridge granite.
Figure 9. Plot of K/Rb versus (a) Rb (ppm) and (b) U (ppm) in the felsic volcanic
samples from the Cumberland Hill Formation.